Surge arresters are known in the most varied embodiments. For example, EP-A-0 642 141 discloses a surge arrester. This known surge arrester has an active part for diverting overvoltage which, in particular, consists of varistor blocks. The active part or the varistor blocks, respectively, are inserted into a prefabricated frame of fiber-glass reinforced polyamide. The known frame has on its sides frame openings through which the diverting elements are inserted into the frame. After the insertion of the electrodes and of the varistor blocks into the frame, the varistor blocks and the electrodes are clamped firmly inside the frame by means of an electrically highly conductive clamping device by which means, on the one hand, the varistor blocks or the active part, respectively, is held firmly in the frame and, on the other hand, a contact pressure for contacting the varistor blocks with one another and/or for contacting the varistor block by means of one of the electrodes is built up. In EP-A-0 642 141, the clamping device, which can have a threaded pin guided in a screw, at the same time forms in each case a connecting electrode or a connecting fixture, respectively, or parts thereof for the surge arrester. In other words, the electrode is displaceable with respect to the frame for the purpose of contacting the varistor block, in such a manner that, by rotating the threaded pin, the electrode initially spaced apart from the varistor block is brought to rest against the varistor block and can be loaded with pressure. The frame with the varistor blocks inserted therein is completely sheathed by a jacket with shields. The jacket with the shields is also called a weather protection housing.
However, in this known surge arrester, a gap is formed between the frame and the varistor block. In the ideal case, this gap is filled by a silicone compound of the weather protection housing. Due to the thermal loading during the operation of the surge arrester and the permeability of silicone for water vapor, water can accumulate especially in this gap. Such accumulations of water can contribute to the failure of the surge arrester.
A further surge arrester is known from EP-A-0 614 198. In this surge arrester, the diverting elements are arranged between connecting fixtures which, in turn, are connected by means of loops. A contact electrode for contacting the varistor block and the varistor blocks are clamped to one another by means of a rotatable pressure screw held in one of the connecting fixtures. By this means, the contact pressure between the varistor blocks to one another and between the contact electrodes and the varistor blocks resting against them is also built up. Each of the loops is produced of wound, fiber-glass reinforced tapes which are embedded in a plastic matrix. The diverting elements and the loops are enclosed completely, and the connecting fixtures are enclosed at least partially, by a cast housing of insulating material provided with shields, which forms a weather protection housing.
A further surge arrester is known from EP-A-0 847 062. In this surge arrester, the clamping device is constructed by a tube of insulating material into which one of the fixtures is screwed at the end. Between the fixtures, a diverting element is again arranged. By screwing the fixture into the tube of insulating material, the fixtures, the tube of insulating material and the diverting element are firmly clamped to one another. Furthermore, this printed document shows that the tube of insulating material may be manufactured of a thermoplastic polymer and a filling material embedded therein such as, for example, glass fibers.
In a further embodiment disclosed in EP-A-0 847 062, of the surge arrester, the tube of insulating material is closed at its ends with lids of insulating material after the assembly of the diverting element and of the fixtures in the tube of insulating material. The same insulating material may be used for the lids and the closing may be performed by means of ultrasonics.
From EP-A-0 372 106, a surge arrester is known in which the diverting elements, heat absorption elements and connecting fixtures are inserted into a prefabricated tube of polyethylene. After the insertion of the diverting elements, heat absorption elements and connecting fixtures, the tube is shrunk by heating in such a manner that the diverting elements, heat absorption elements and connecting fixtures are firmly pressed against one another.
From EP-A-0 393 854, a surge arrester is known which has gas outlet openings in order to prevent any bursting apart in the case of a fault.
Further surge arresters are known from WO 97/32382. A first surge arrester known from WO 97/32382 has reinforcing strips which are embedded in a plastic matrix for reinforcement in the axial direction of the surge arrester.
Another surge arrester also disclosed in WO 97/32382 has a connecting element made of an insulating material which holds together electrodes and varistor blocks. The connecting element has a basic layer of a resin material. Furthermore, the connecting element has one or more outer layers which are also of resin material. Relatively short fiber bundles are mixed into the resin material of the outer layers.
Surge arresters which may be used in medium- and high-voltage grids are intended to meet the specifications of, among other things, the IEC 60099 standard. However, known surge arresters are susceptible to gas formation inside the active part under overload of the diverting element, that is to say if the active part with the varistor has absorbed too much energy due to too high a temporary overvoltage or too high a line discharge. If the gas cannot escape from the surge arrester, this leads to an explosion of the surge arrester. This represents a danger for the plant itself in which the surge arrester is arranged and for persons who are occupied in the immediate vicinity of the surge arrester.